Conventional reduction of dinitro aromatic compounds to aromatic diamines is practiced with hydrogen as a reducing agent and noble metal catalysts. Large amounts of hydrogen are consumed in the process and the noble metal catalysts are expensive.
D. M. Gold in U.S. Pat. No. 3,255,252 issued June 7, 1966 discloses reacting hydrogen sulfide and organic mono- and dinitro compounds in liquid phase (see col. 3, line 54) to form amines. Specifically in the working example, nitro benzene, in a heated autoclave under pressure is reacted with hydrogen sulfide in the presence of a silica/alumina catalyst to form aniline. Other heterogeneous catalysts disclosed but not exemplified in quantitative examples are associations of silica and various amphoteric metal oxides and particularly those of alumina, thoria and zirconia, e.g., active clay, synthetic alumina-silica, thoria-silica and zirconia-silica catalysts. It is stated that related conversion "can be realized," e.g., "nitrotoluenes" to "nitrotoluidenes, toluene diamines," but no quantitative examples are given.
J. J. Wise in U.S. Pat. No. 3,253,038 issued 1968 similarly discloses reduction of specifically only nitrobenzene, with hydrogen sulfide, preferably in the vapor phase at 300.degree. C. over a crystalline alumina zeolite as catalyst (especially in the sodium form). It is indicated that in aromatic compounds containing two or more nitro groups, one of these can be selectively reduced by the subject process.
G. E. Etzel in J. Phys. Chem. Vol. 32 page 852 describes investigations relating to the catalytic activity of titania in the reduction of nitro compounds with hydrogen, and cites earlier studies using catalysts such as iron, cobalt, alumina, and numerous others.
Murata in Organic Chemistry (Japan) Volume 35, 61 (1977) discloses the reduction of aromatic nitro compounds with hydrogen sulfide in liquid phase.
The prior art might suggest, and we have confirmed, that when gaseous hydrogen sulfide is employed as the reducing agent for catalytic reduction of dinitroaromatics, the proportion of diamine in the resulting product is low. Moreover, we have ascertained that using catalysts representative of known reduction catalysts, the activity observed for diamine production declines rapidly.
We have now found conditions under which hydrogen sulfide is effective for reduction of two or more nitro groups in di- and/or poly nitroaromatics to amino groups and remains so for extended periods of running.